In a prior art press of this kind a wedge is provided as the thrust piece, which is guided with possibility of displacement of a vertical guide in parallel with the direction of movement of the ram traverse, driven by a toggle-lever drive, and of the linkage traverse and can be inserted transversely to such a direction of movement to a certain extent between the ram traverse and the linkage traverse; the greater the required residual stroke of the ram for the secondary compression of the casting the greater the depth of the insertion. In each working cycle of the known press the wedge is pressed downward by the ram traverse and, during this, it should press the linkage traverse downward initially by an identical amount. This means that the wedge may not slide out laterally from between the ram traverse and the linkage traverse. However, as soon as the linkage traverse reaches its final press position, a sliding surface formed on the lower side of the wedge contacts a stationary cross-bar and the wedge must then overcome a spring force which supports it and slide out laterally between the ram traverse and the stationary cross-bar, so that the ram traverse together with an upper die attached thereon may perform a downward residual stroke in which the linkage traverse and a mold attached thereon no longer participate. One is faced at least with difficulties when the spring force that supports the wedge laterally is to be selected so that, together with the friction effective between the wedge and the two-above mentioned traverses, it prevents a premature lateral deflection of the wedge, on the one hand, and, on the other hand, however, so that, together with the friction effective between the ram traverse, the stationary cross-bar and the wedge, it is not capable of preventing the lateral deflection of the wedge when it reaches the cross-bar.
In another prior art press of this type, the motions transmitted from the withdrawal cam and return stroke cam to the linkage are coupled through an infinitely adjustable differential gearing. The latter is associated with two bevel-gear segments that are mounted on the pivot pin of the lever pivoted on the press frame and connected to the linkage, the segments engaging a bevel pinion mounted between the segments on the lever. A bevel gear segment possesses a radial serration concentric with the pivot pin and meshing with a spur gear segment. The latter is attached to a shaft mounted in the press frame at a distance and parallel to the pivot pin of the lever, the shaft carrying a cam lever comprising a cam follower roll. The other bevel gear segment possesses a radial connecting link guide, which is engaged by a sliding block mounted on a carriage. The carriage can be displaced with the possibility of infinite variation along a lever that is attached on a second shaft and which is mounted in the press frame likewise at a distance and parallel to the pivot pin, coaxially to the first shaft which is mentioned above. Also, the second shaft carries a cam lever on which a cam follower roll is mounted. The two cam follower rolls are associated in each case with one of the two cams attached to the main shaft of the press. The lever connected to the linkage represents a balancing between the swinging movements of the two bevel-gear segments. When the two bevel-gear segments perform swinging movements of the same size and direction, such movements are transmitted in the full magnitude to the lever. By shifting the carriage together with the sliding block that engages the guide of a bevel-gear segment, the magnitude of the swinging movements of such a bevel gear segment can be varied at the given shape of the cam associated with the segment. Owing to the balancing effect of the bevel pinion, this produces the possibility of varying the total effect exerted by the two cams on the lever connected to the linkage. However, the effects of the two cams cannot be separated from each other, which makes it difficult to adjust the strokes of the linkage to the strokes of the ram and, since press tools vary, it is not possible to avoid a frequent replacement of the cams.